Optical multi/demultiplexer device, optical wavelength selective filter and method of making filter

ABSTRACT

An optical wavelenth selective filter on an optical waveguide and a method of making the same is provided. The filter is able to transmit a first predetermined band of wavelengths and to reflect a second predetermined band. The filter further includes a plurality of transmissive couples and reflective couples in series, with each of the couples including a first zone of high refractive index and a second zone of low refractive index adjacent to each other so as to form a structure with a high gap modulated refractive index.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/EP01/12419, filed Oct. 26, 2001, the content of which isincorporated herein by reference.

Present invention regards an optical multi/demultiplexer device forwavelength division multiplexing optical signals.

For wavelength division multiplexing, or WDM, optical signals, aplurality of mutually independent optical signals has to be sent along aline, comprising optical fibers or waveguides, by means of multiplexingin the optical wavelength domain; the transmitted signals can be eitherdigital or analog, and they are distinguished from each other in thateach of them has a specific wavelength, separate from that of the othersignals.

To implement this WDM transmission along a line, specific wavelengths ofpredetermined amplitude, termed “channels” in the following text, haveto be assigned to each of the signals at different wavelengths. Thesechannels, each identified in the following text by a wavelength value,called the central channel wavelength, have a certain spectral amplitudearound the central wavelength value, which depends, in particular, onthe characteristics of the signal source laser and on the modulationimparted to this to associate a data element with the signal. Typicalvalues of spectral amplitude of the channels are 200 GHz or 100 GHz (ITUband). With this spectral amplitude the gap between one channel and oneother channel is 1.6 nm or 0.8 nm.

In case of spectral amplitude substantially higher than 200 GHz the WDMsignal is known as CWDM signal or “coarse WDM” signal.

Currently, in the telecommunication field, optical technology is mainlyused for long-distance transmission of optical signals using the knownproperties of wide band provided by optical fibers. On the contrary, themost used technology for distributing signals to a plurality of users(such as for example, analogue and/or digital television and/ortelephone signals) and for transmitting digital data between electronicapparatuses (such as for example, the Personal Computers of a LANnetwork) makes use of electric cables such as, for example, coaxialcables or those made up of copper pairs.

Nevertheless, electric cables have a relatively narrow band, and theyare becoming a bottleneck with respect to the band of signals to betransmitted. Moreover, they present problems of electromagneticinterferences, of impedance matching, and they are difficult to beintroduced into the appropriate raceways of a building since they arestiff. In addition, being bulky, they significantly reduce the number ofcables that can be inserted into a raceway. Moreover, due to electricalsafety requirements, they require the arrangement of separate racewaysfrom those used for distributing electric energy.

Thus, the research is investigating the possibility of using optics notjust in the long-distance transmission of signals, but also in thesignal distribution networks from a common branch point to a pluralityof user apparatuses. In fact, optical-fiber cables are suitable to beinserted into the appropriate raceways of a building since they are nottoo bulky, they are flexible, light, free from electromagneticinterferences, and their bending loss is very low. Moreover, they aresuitable for being inserted into the same raceways used for distributingelectric energy. Additionally, optical fibers potentially have a verywide band, low attenuation values, and they are transparent to the bitrate, to the format and to the code of transmission.

Moreover, among the various types of optical fiber, single-mode opticalfibers are preferable since they are much less sensitive to bendinglosses, and they are less expensive, more rugged, with lower absorptionlosses than multimode fibers; they are suitable to be used for WDM orCWDM transmission, and they have a wider band, thus making a signaldistribution network easily upgradeable.

U.S. Pat. No. 5,911,019 describes an optical fiber communication networkcomprising a fiber distribution node that is fed with a feeder opticalcable from a central office; a plurality of network unit act to convertthe signal from a fiber optic signal to an electrical signal. Said fiberdistribution node comprises multiplexer/splitters which distributecommunication signals to said network units. Finally, electrical cablesfor connecting said network units with a plurality of living units areused. Typically two bands of the frequency spectrum with wavelength of1300 nm and 1550 nm are used to transmit individual voice channel backand forth from the living units to the central office. The band in 1550nm range is used to transmit a single voice channel to the living unitsto the central office and a band in 1300 nm range is used in the reversedirection or vice versa.

An optical local area network is described in WO0150644.

Applicant of present invention has observed that in an optical localarea network a WDM signal may be used for distributing a plurality oftelevision and/or telephone and/or Internet digital or analog signalcoming from a long-distance transmission via optical fiber to aplurality of electronic user apparatuses. For example, one channel ofsaid WDM signal can be used for a Internet digital signal, anotherchannel can be used for a television signal etc. In this way, eachsignal has a large band available. For correct reception of thesetransmission signals (WDM), it is necessary to provide a separationbetween the signals at different wavelengths, for directing them to thecorresponding users of the network (optical local area networks). Ademultiplexer device provides to realize said separation between thesignals. Moreover, the signals of user apparatus like computers(Internet digital or analog signals) and telephone signals, arebi-directional. In this case the demultiplexer device must be used alsolike a multiplexer device.

To realize an optical multi/demultiplexer device many technologies areknown. One technology exploits phase shifted Bragg filters. Thosefilters are conveniently used for the selection of single narrow-bandchannels and also for multichannel wavelength demultiplexing. TheArticle “Phase-Shifted Bragg grating filters with improved transmissioncharacteristics” published on Journal of Lightwave Technology No 12December 1995 describes a filter with a quarter wave shifted Bragggrating having an extremely narrow transmission peak in the center ofthe stop band but its shape is not suitable for system design. Byintroducing several phase shifted regions and properly choosing theirlocation and magnitudes, the transmission spectrum can be tailored intoa nearly rectangular shape.

The Applicant has observed that in said article the filter is applicablein high rate optical systems, a distance between a channel and anadjacent channel of 0.4 nm (3 dB bandwidth). Generally, the gap in highrate optical transmission system actually is around 200 GHz or 100 GHz(ITU band), with a gap in wavelength between one channel and one otherchannel of 1.6 nm or 0.8 nm.

Optical local area networks would use low cost devices and components,for example low cost transmission lasers to transmit optical signalstrough the network, optical multiplexers, splitters and electronicdevices. Usually, said lasers are significantly different fromtransmission lasers of long-haul optical telecommunication systems whichare lasers at high cost. The lasers of long-haul opticaltelecommunication systems are temperature stabilized because substantialfluctuations of transmission wavelengths are detrimental. In DWDM(Dense-WDM) telecommunication system the channels are separated fromeach other less than 1 nm. Devices for stabilizing temperature andwavelength in lasers are at high cost.

Said lasers for an optical local area network preferably transmitchannels which are separated from each other more than 1 nm. Preferably,a typical laser for optical local area network is a vertical cavitylaser which transmits wavelengths in the range of 800–900 nm. Thefluctuations of the central wavelength of said laser may be more than0.5 nm.

The Applicant has addressed the problem of providing a CWDMmulti/demultiplexer that is preferably applicable to optical WDM signalsused in optical local area networks. In particular, the Applicant hasaddressed the problem of separating optical WDM signals having a largespectral amplitude. Each channel transports information that must bedetected by a receiver. The CWDM multi/demultiplexer device should havea sufficiently flat band around the central wavelength value of eachchannel in a way to maintain the detectability of the informationtransported of each channel. Moreover, the Applicant has addressed theproblem of providing a CWDM multi/demultiplexer at low cost, with smalldimensions and possibly able to being easily coupled with electronicdevices.

The Applicant has found that a filter that is able to separate channelsof a CWDM optical signal with a large and flat spectral response may berealized by means of optical structures like gratings with a high gapmodulated refractive index. These filters are able to separate opticalWDM signals with a large band.

An optical structure with high gap refractive index is a structurecomprising at least a first zone with a first refractive index and atleast a second zone with a second refractive index, said first zone andsecond zone being adjacent each other, in wihch the gap between saidfirst refractive index and said second refractive index is more than0.1. In particular, said first zone is a zone with high refractive index(for example a block of silica having a refractive index of about .1.5),said second zone is a zone with low refractive index (for example a cutrealized on a waveguide, air has a refractive index of about 1).

A optical structure with high gap modulated refractive index is astructure comprising a plurality of zones at high refractive index and aplurality of zones at low refractive index, between two consecutivezones at high refractive index one zone at low refractive index beingformed, in which at least two of said plurality of zones at highrefractive index have different dimensions from each other.Advantageously, at least two of said plurality of zones at lowrefractive index have different dimensions from each other.

The succession or alternation between zones at high refractive index andzones at low refractive index generates a plurality of transmissivecouples and a plurality of reflective couples, both said transmissivecouple and said reflective couple comprising one of said high refractiveindex zones and one of said low refractive index zones. In particular,each transmissive couple transmits an optical beam in a predeterminedtransmissive wavelength band and each reflective couple reflects aoptical beam in a predetermined reflective wavelength band.

Preferably, said optical structure is substantially a symmetricstructure with respect to the light propagation direction, that is, thefirst half of said structure is symmetric with respect to the secondhalf.

The entire structure comprises a plurality of transmissive couples and aplurality of reflective couples in series. Preferably, the dimension ofthe transmissive couples are different from each other. Moreover, thedimension of the reflective couples may be diferent from each other.

Said alternation of transmissive couples and reflective couplesgenerates a sufficiently wide transmissive band (pass band) between twoadjacent reflective band (stop band).

The spectral response of said structure is the product of the spectralresponse of all couples (trasmissive and reflective). In this way theglobal spectral response is steeper than the spectral response of eachsingle couple.

An “apodized” structure, in which between a first transmissive coupleand a second transmissive couple at least a reflective couple isdisposed, generates a more flat transmissive band (pass band) than in astructure in which consecutive transmissive couples are disposed. Theincrease of the width of successive blocks in an apodized structurefurther contributes to obtain a flat pass band. The increase of thenumber of consecutive reflective couples generates is a further step toobtain a flat pass band.

In particular, the Applicant has realized a multi/demultiplexer deviceon a semiconductor substrate comprising waveguides on which said filtersare provided. Said filters comprise a resonant structure formed by aplurality of said zones with high refractive index and a plurality zoneswith low refractive index disposed in predetermined positions of saidwaveguide.

A first aspect of the present invention regards an opticalmulti/demultiplexer device comprising

-   -   a substrate,    -   a plurality of waveguides on said substrate,    -   at least a wavelength selective filter realized on one of said        plurality of waveguides,    -   said at least one filter being able to transmit a first        predetermined band of wavelengths and to reflect a second        predetermined band of wavelengths,        wherein said at least one filter comprises:        at least a first zone with a high refractive index and at least        a second zone with a low refractive index, said first zone and        second zone being adjacent each other and forming a plurality of        transmissive couples providing said first predetermined band and        a plurality of reflective couples providing said second        predetermined band disposed in series to each other, in a way to        form a structure with high gap modulated refractive index.

Preferably, said zones with a low refractive index are realized by meansof a plurality of transverse cuts provided on said waveguides and saidzones with a high refractive index are blocks formed between twoconsecutive cuts.

Preferably, each couple comprises one cut and one block.

Preferably, between a first transmissive couple and a secondtransmissive couple at least a reflective couple is disposed.

Preferably, in the first half of said filter the blocks of saidtrasmissive couples increase in width in the direction of propagation ofthe light.

Preferably, in said first half consecutive reflective couples increasein number in the direction of propagation of the light.

Preferably, said reflective couples have the same width.

Preferably, a second half of said filter has an opposite correspondencewith the first half.

In particular, said second half of the structure is symmetric of saidfirst half.

In particular, in said reflective couples the sum of the width of saidcut and the width of said block is substantially an odd multiple of aquarter of the central wavelength of said second predetermined band.

In particular, in said transmissive couples the sum of the width of saidcut and the width of said block is substantially an even multiple of aquarter of the central wavelength of said first predetermined band.

Advantageously, at least one of said plurality of wave guides comprisesa ridge. In particular, said substrate is a semiconductor substrate

A further aspect of present invention regards an optical wavelengthselective filter realized on an optical waveguide, said filter beingable to transmit a first predetermined band of wavelengths and toreflect a second predetermined band, characterized in which itcomprises:

at least a first zone with a high refractive index and at least a secondzone with a low refractive index, said first zone and second zone beingadjacent each other and forming a plurality of transmissive couplesproviding said first predetermined band and a plurality of reflectivecouples providing said second predetermined band disposed in series toeach other, in a way to form a structure with high gap modulatedrefractive index.Preferably, said zones with a low refractive index are realized by meansof a plurality of transverse cuts provided on said waveguides and saidzones with a high refractive index are blocks formed between twoconsecutive cuts.

A further aspect of present invention regards a method for realizing anoptical wavelength selective filter on a substrate comprising thefollowing steps:

providing a waveguide in a optical conductive material on saidsubstrate, providing a plurality of cuts on said waveguide, in a way toform a plurality of transmissive couples providing a first predeterminedband of wavelengths and a plurality of reflective couples providing asecond predetermined wavelength band disposed in series to each other.

Further features and advantages of the present invention will appearmore clearly from the following detailed description of a preferredembodiment, made with reference to the attached drawings. In suchdrawings:

FIG. 1 shows schematically an optical local area network installed in abuilding according to present invention.

FIG. 2 shows schematically a distribution unit arranged in a cellar orbasement of the building of FIG. 1.

FIG. 3 shows a multi/demultiplexer device of the network of FIG. 1according to present invention.

FIG. 4, illustrates a lateral view of a portion of saidmulti/demultiplexer device in which are shown a plurality of layersaccording to present invention.

FIG. 5, illustrates one view of a wavelength selective filter accordingto present invention.

FIG. 6 a, illustrates one view of a wavelength selective filter.

FIG. 6 b, illustrates one graph of the spectral response of filter ofFIG. 6 a.

FIG. 7 a, illustrates one view of a wavelength selective filteraccording to present invention.

FIG. 7 b, illustrates one graph of the spectral response of filter ofFIG. 7 a

FIG. 8, illustrates one simulation view of a wavelength selective filteraccording to present invention.

FIG. 9, illustrates one graph of the spectral response of filter of FIG.8.

FIG. 10 shows a further embodiment of a multi/demultiplexer device ofthe network of FIG. 1 according to present invention.

FIG. 11 shows a graph of the width of a block and a cut for a reflectivecouple.

FIG. 1 shows schematically an example of an optical local area network,installed in a building 1, comprising an external optical cable 2, forexample incoming from an external system of telecommunication, which isconnected to a distribution unit 3 exemplarily arranged in a cellar orbasement of said building 1 or at another convenient location in orclose to the building.

In general, optical local area networks can be used, for example, fordistributing a plurality of television and/or telephone and/or Internetdigital or analog signals coming from a long-distance transmission viasatellite and/or via coaxial cable and/or via optical fiber and/orthrough the air to a plurality of electronic user apparatuses.

In FIG. 1, the building 1 comprises a plurality of living units A1–A6;for example said living units are subdivided in three floors. Each ofsaid living units is provided with a suitable raceway apt to receive atleast one connection cable C1–C6 carrying said digital or analogsignals.

The term living unit as used herein is somewhat of a misnomer as itincludes any subscriber who receives and/or sends services from/to thenetwork.

Said connection cable may be an optical cable or an electrical cable oran hybrid cable in which both optical signals and electrical signals arepresent.

As shown in FIG. 2, said distribution unit 3 comprises an opticalmulti/demultiplexer device 4 and a distribution device 5. Saidmulti/demultiplexer device 4 separates from each other the channels, orgroup of channels, of the WDM signals incoming from the external opticalcable 2, and said distribution device 5 splits each of the singlewavelength channels, or each group of channels, preferably in a numberof signals corresponding to the number of living units and couples thesignals to a plurality of said connection cables. An example of saidconnection cable is described in patent application U.S. Pat. No.5,978,536.

Preferably, said distribution device 5 comprises a beam splittersuitable to route a digital optical signals coming from themulti/demultiplexer device 4 on input/output optical ports. Saidinput/output ports are advantageously bi-directional ports.

The distribution device 3 alternatively may comprise an opto-electricalconversion unit (not shown) in which the optical signals from themulti/demultiplexer device 4 may be converted in electrical signals.

In this embodiment, the distribution device 5 distributes digitalsignals to a plurality of users, for example, according to a 100 Mbit/sFastethernet™ protocol. Said signals arrive in optical form to saidmulti/demultiplexer device 4. Then, they are converted intocorresponding electric signals by said opto-electrical conversion unit.Moreover, the distribution unit is suitable to select, among theelectric signals, the digital signal intended for each user (forexample, according to a 10 Mbit/s Ethernet™ protocol) and to send it toa corresponding opto-electronic converter. Said converter converts thedigital electric signal intended for the user into a correspondingoptical signal and sends it to the corresponding user apparatus throughsaid connection cable.

An optical local area network advantageously uses low cost transmissionlasers to transmit optical signals through the network. Usually, saidlasers are significantly different from transmission lasers of long-hauloptical telecommunication systems which are lasers at high cost. Thelasers of long-haul optical telecommunication systems are temperaturestabilized because substantial fluctuations of transmission wavelengthsare detrimental. In DWDM telecommunications system the channel areusually separated from each other less than 1 nm.

In optical local area networks preferably channels are separated fromeach other more than 1 nm. More preferably, said channels are separatedfrom each other more than 1.6 nm.

A tipycal laser for optical local area networks is a vertical cavitylaser which transmits wavelengths in the range of 800–900 nm. However,the multi/demultiplexer device of the present invention canalternatively be designed to separate channels which are in a differentrange of wavelengths (for example around 1300 nm).

Said lasers need be temperature stabilized and the central transmissionwavelengths are free to fluctuate, for example under the effect oftemperature changes, of more than 0.5 nm. The multi/demultiplexer deviceof the present invention has a large band to overcome these fluctuationsof the central wavelength.

The multi/demultiplexer device 4 as shown in FIG. 3, comprises asubstrate 41, preferably realized in a semiconductor material, forexample silicon. On said substrate a plurality of layers is superposed.In particular, on the top of said substrate 41 a first layer 42 issuperposed. Preferably, said first layer is of silica (silicon dioxide).On the top of said first layer 42 a second layer 43 is arranged.

Preferably, said second layer is of silica nitride (SiOxNy). On the topof said second layer 43 a third layer 44 is arranged. Preferably, saidthird layer is of silica.

Said first layer and said third layer are respectively a bottom claddingand a top cladding of a waveguide, and said second layer represents acore of said waveguide. All the material are optical transmissivematerials.

On the surface of said third layer 44 a plurality of ridges is provided.The shape of said ridges determines the lateral confinement and thepropagation direction of the optical beam which travels along thewaveguide. In FIG. 4, a portion P, in a lateral view, of saidmulti/demultiplexer device of FIG. 3 is illustrated, in which saidplurality of layers is shown.

In the example of FIGS. 3 and 4, said waveguides with ridges areprovided. A different kind of waveguides may be provided, for example aburied waveguide in which a guided element is buried into a layer forexample of silica Said guided element works like a ridge and itdetermines the lateral confinement and the propagation direction of theoptical beam which travels along the waveguide. Another used waveguideis an arrow waveguide like a photonic crystal in which on a opticalsubstrate a plurality of holes are provided. The position of said holesdetermines a waveguide on said optical substrate.

In particular, in FIG. 3 a first ridge 45 a second ridge 46 a thirdridge 47 and a fourth ridge 48 are provided. Preferably, the first ridgecrosses the entire layer, one end of said ridge being an input port 451for a WDM signal.

Along said first ridge 45 a first filter 452 is provided. The opticalbeam incoming from said input port 451 is partially reflected by saidfirst filter and partially transmitted to a first output port 453 at theother end of said first ridge 45. The reflected beam is directed to oneend of said second ridge 46. Along said second ridge 46 a second filter462 is provided. The optical beam incoming from the end of said secondridge 46 is partially reflected by said second filter 462 and partiallytransmitted to a second output port 463 at the other end of said secondridge 46. The reflected beam is directed to one end of said third ridge47. Along said third ridge 47 a third filter 472 is provided. Theoptical beam incoming from the end of said third ridge 47 is partiallyreflected by said third filter 472 and partially transmitted to a thirdoutput port 473 at the other end of said third ridge 47. The reflectedbeam is directed to one end of said fourth ridge 48. Along said fourthridge a fourth filter 482 is provided. The optical beam transmitted tosaid fourth filter 482 is output to a fourth output port 483.

Said filters are preferably optical wavelength-selective filters. Inparticular, each filter is able to reflect a predetermined wavelength ora predetermined wavelength interval in a predetermined bandwidth. Theremaining wavelengths in the predetermined bandwidth are preferablytransmitted across the filter.

In the example of FIG. 3 said filters along said ridges are provided.Equivalently, said filter on other kind of waveguides as above cited,may be provided.

Said filter may be realized by means of optical structures like gratingswith a high 10 gap modulated refractive index.

A optical structure with high gap refractive index is a structurecomprising at least a first zone with a first refractive index and atleast a second zone with a second refractive index, said first zone andsecond zone being adjacent each other, in which the gap between saidfirst refractive index and said second refractive index is more than0.1. In particular, said first zone is a zone with high refractive indexsaid second zone is a zone with low refractive index.

A optical structure with high gap modulated refractive index is astructure comprising a plurality of zones at high refractive index and aplurality of zones at low refractive index, between two consecutivezones at high refractive index one zone at low refractive index beingformed, in which at least two of said plurality of zones at highrefractive index have different dimensions from each other.

Advantageously, at least two of said plurality of zones at lowrefractive index have different dimensions from each other.

Said optical structure is preferably a symmetric structure with respectto the light propagation direction, that is, the first half of saidstructure is symmetric with respect to the second half.

Said zones at high refractive index and said zones at low refractiveindex may be realized by means of material of different opticalcharacteristic. In particular, said zone at high refractive index may bea silica based material refractive index≅1.5), said zone at lowrefractive index can be made by means of a cut realized in said silicamaterial (air refractive index=1). Alternatively, said zones may berealized by means of other material, provided that between a zone athigh refractive index and a zone at low refractive index a high gap ofrefractive index is present. In particular in the example illustrated inFIGS. 3 and 4 and 5 the zones at low refractive index are realized bymeans of cuts. In fact, a cut is easily realized on a ridge. In otherkind of waveguides said zones at low refractive index may be realized byinserting a piece of a predetermined material.

In FIG. 5, along said ridge a plurality of cuts are provided. Said cutshave a depth preferably equal to the depth of the ridge and the threelayers. The cuts have a transverse predetermined direction of cuttingwith respect to the length direction of the ridge along which they aredone. The cuts may be realized for example by means of electron beamlithography and by a successive etching. This is a known technique ofthe microelectronic technical field. Other techniques can be used formaking the cuts.

For example, in FIG. 5 a first cut 4521, a second cut 4522 and a thirdcut 4523 are illustrated. Said cuts may be emptied to create vacuum ormay be filled by air, another gas or a liquid and said cuts generate atleast a resonant structure like a grating; in particular, a structurewith a modulated refractive index is provided along the opticalwaveguide. In fact, an index of refractive step or gap arises betweenthe refractive index of the layers (for example silica refractiveindex≅1.5) and that of air (refractive index≅1). Between two consecutivecuts a block is formed.

Such filter reflects some wavelengths in a predetermined band (stopband) and transmits other wavelengths in a predetermined band (passband). Preferably, the direction of cutting is chosen to direct saidreflected beam at wavelengths in said stop band to another ridge on thesubstrate.

Example of structures generated by cutting said ridges are schematicallyillustrated, in a lateral view, in FIGS. 6 a and 7 a.

The two figures are positioned in a way to facilitate the comparisonbetween them.

In particular, the structures are disposed with respect to a couple ofCartesian axes X,Z, where X indicates the position along the lightpropagation direction, Z indicates the position in the depth directionof the cuts.

In particular, in FIG. 6 a a structure with a plurality of transversecuts, each cut preferably having a same width, is illustrated. This kindof structure is a resonant structure and permits to have a transmissionspectral response in a predetermined wavelength band as shown in FIG. 6b. In particular, the graph of FIG. 6 b illustrates a reflection band SBand a transmission band PB in a domain of wavelengths λ. The width ofthe cuts and the distance between a cut and the next cut determine thecentral wavelength λ_(SB) of the stop band SB, the central wavelengthλ_(PB) of the pass band PB and the width of said SB and PB.

In FIG. 6 a a cell element C comprises a cut and a block of layers. Inparticular, the length of the cell d_(c) is:d _(c) =d _(cut) +d _(block).

For the above definitions it is intended that the dimensions of the cutsand the blocks are calculated with respect of the refractive index ofthe material of the blocks and the cuts (for example SiO2 for blocks andair for cuts).

In particular:d _(cut) =d _(cut) ′/n _(eff cut)d _(block) =d _(block) ′/n _(eff block)where d_(cut)′ and d_(block)′ are respectively the real dimension of thecut and the block and n_(eff cut) n_(eff block) are respectively theeffective refractive index of the cut and the block.

The value of the central wavelength λ_(SB) of the stop band SB dependsfrom d_(c), in particular d_(c) is substantially a multiple of λ_(SB)/2.

A cell element has a length dc substantially multiple of λ_(SB)/2.

The above condition is verified for example if:d _(cut)=λ_(SB)/4n _(air) andd _(block)=λ_(SB)/4n _(block)

From the above relations it is possible to choose the dimension of thecuts and the distance between two adjacent cuts once the stop band SBfor the filter has been selected.

From the graph of FIG. 6 b, the Applicant has observed that the passband PB of the filter of FIG. 6 a comprises some undesirable peaks.Moreover, by introducing cuts all of the same dimension (FIG. 6 a) it isnot possible to choose the width of the stop band independently from thewidth of the pass band.

In FIG. 7 a, an example of an “apodized” structure is illustrated. Inparticular, said structure comprises a plurality of cuts inpredetermined positions of the ridge. The structure of the filtercomprises at least one defect which is represented by a differentdimension of at least one of said blocks. Preferably, said blocksgenerated by said cuts are of different dimension and the cuts are notequidistant from each other.

This means that the variations of the effective refractive index are notstrictly periodic; but the variations of refractive index varies betweenat least two values correspondent to the refractive index of cuts andblocks. The pitch of said variation is variable along the filter.

This kind of structure generates a new pass band which is comprised intothe stop band of the filter of FIG. 6 a (see graphic of FIG. 6 b). Ithas to be noted that the graphs of FIG. 6 b and FIG. 7 b havecorrespondent vertical axis and the same wavelength (horizontal) scale.The structure of FIG. 7 a comprises a plurality of substructures eachcomprising at least a cut and a block. Said substructures are disposedadjacent to each other.

For the purpose of the present invention a reflective substructure orreflective couple comprises one cut and one block, in which the sum ofthe width of said cut and the width of said block is substantially aneven multiple of λ_(SB)/4, where λ_(SB) is the central wavelength of thestop band or reflective band.

For the purpose of the present invention a transmissive substructure ortransmissive couple comprises one cut and one block, in which the sum ofthe width of said cut and the width of said block is substantially anodd multiple of λ_(PB)/4, where λ_(PB) is the central wavelength of thetransmissive band or pass band. Said block of said transmissive couplecomprises the above-cited defect.

Each one of said substructures is also a resonant structure having areflection band and a transmission band respectively centered in λ_(SB)and λ_(PB) as above defined.

For the purpose of the present invention an “apodized” structurecomprises a plurality of transmissive couples and a plurality ofreflective couples, in which between a first transmissive couple and asecond transmissive couple at least a reflective couple is disposed.

Preferably, a first half of the structure comprises at least a first anda second transmissive couple in which the width of the block of thesecond transmissive couple is bigger than the width of the block offirst transmissive couple.

In other words in said first half of the filter in the direction ofpropagation of light the width of the block of successive transmissivecouples increase.

Moreover, in said first half the dimensions of the reflective couplesmay be different each other. Advantageously, in said first half of thestructure consecutive reflective couples increase in number.

In particular, for the purpose of the present invention “increase innumber” means that in said first half of the filter in the direction ofpropagation of light the number of consecutive reflective couples may bethe same or may be increased.

Preferably, said reflective couples have the same width.

Preferably, a second half of the structure has an oppositecorrespondence with said first half. For the purpose of the presentinvention “opposite correspondence” means that the first half of thestructure has the same configuration about the number of the trasmissivecouples and the reflective couples and about the disposition in sequenceof them.

Preferably said second half of the structure is symmetric to said firsthalf. That is, the modulation of refractive index in said first half isopposite proportional to the modulation of the refractive index in saidsecond half. In particular a symmetric structure has the first halfspecular to the second half.

As above described the structure comprises a plurality of substructuresor couples (reflective and transmissive) in series; each of saidsubstructures is a filter itself, and the structure in total comprisesmany filters in series. Said alternation of transmissive couples andreflective couples generates a sufficiently wide transmissive band (passband) between two adjacent reflective band (stop band).

Moreover, the spectral response of the structure in total in the passband and also in the stop band is more stepped than the spectralresponse of a single filter. In other words the number of defects of thetransmissive couples determines the stepness of the pass band and thestop band of the structure. The transmission couple having the biggestdefect of the entire structure determines substantially the width of thepass band. In fact, the width of the defects is in opposite relationshipwith the bandwidth of the pass band.

An “apodized” structure, in which between a first transmissive coupleand a second transmissive couple at least a reflective couple isdisposed, generates a more flat transmissive band (pass band) than in astructure in which consecutive transmissive couples are disposed. Saidincrease of the width of successive blocks in an apodized structurefurther contributes to obtain a flat pass band. Said increase of thenumber of consecutive reflective couples is a further step to obtain aflat pass band.

Advantageously, the symmetric structure generates a symmetric pass bandwith respect to the central wavelength λ_(SB) of the pass band.

In the example of FIG. 7 a the filter comprises five defects D1–D5. Inthe first half of said filter, between a defect and the next defect aincreasing number of cuts is provided and the width of the defectsincreases. In the second half of said filter, between a defect and thenext defect a decreasing number of cuts is provided and the width of thedefects decreases.

The complete structure of FIG. 7 a, comprises a first substructure and asecond substructure which are symmetric with each other. In particular,the structure is symmetric with respect to a vertical axis Y. Saidvertical axis Y is in the middle of said third block D3.

Said first block D1 has preferably a width d1=m₁ λPB/2, said secondblock D2 has preferably a width d2=m₂ λ_(PB)/2, said third block D3 haspreferably a width d3=m₃ λ_(PB)/2, said fourth block D4 has preferably awidth d4=λ_(PB)/2, said fifth block D5 has preferably a width d5 m₅λ_(PB)/2, where m1≦m2≦m3≧m4≧m5 are integers. Preferably, m1=m5 andm2=m4. Generally, the choice of the parameter m determines the width ofthe new pass band PB′ generated. Advantageously, by varying the value ofm1–m5 it is possible to determine the flattened band of the pass band.

In FIG. 7 b, a graph of the spectral response in wavelength of thefilter of FIG. 7 a is shown. The new pass band PB′ is centered on awavelength λ_(PB′) which is correspondent to the central wavelengthλ_(SB) of the stop band of FIG. 6 b. It has to be noted that thedimensions of the cuts of the examples of FIG. 6 and FIG. 7 are thesame.

This new pass band PB′ is substantially flat, and also the new stop bandSB′ is substantially flat. This means that both the reflected channelsand the transmitted channels by the filter maintain substantially thesame shape. Each signal which is associated with a channel reflected ortransmitted is substantially not affected by distortions.

The cuts are regions in which the light is not guided; to the contrarythe light into the blocks is guided. The width of such cuts maydetermine losses of the optical power of the channels. At wavelengthsaround 1500 nm the cuts may be around 360–380 nm. In this case, thelosses may be reduced by reducing the dimension of the cuts and byincreasing the dimensions of correspondent block into the same couple.

For example in FIG. 11 a graph of the dimension of a block and a cutwhich give a reflective couple is shown. In particular, four curves areshown. A first curve C1 is with d_(cut)+d_(block)=λ/2 and with arefractive index of the material (block) n1=1.5. Said curve issubstantially a straight line. A second curve C2 is with a refractionindex of the material (block) of n2=1.45. A third curve C3 is with arefractive index of the material (block) of n3=3. A fourth curve C4 iswith a refractive index of the material (block) of n4=3.33.

Said curves C2, C3 and C4 show how it is possible to compensate thevariation of the width of the cut with another variation of the width ofthe block. The point P is the point in which the resonance of the filteris complete that is the condition d_(cut)+d_(block) is exactly amultiple of λ/2, but the curves show that a condition of substantialresonance may be found with different dimension of cuts and blocks.

EXAMPLE

The Applicant has carried out simulations of one filter of amulti/demultiplexer device with the configuration shown in FIGS. 3–4. Inparticular, in the simulation the dimension of the layers and of theridges are:

-   Bottom cladding 42 height=8.57 μm.-   Core 43 height=896 nm.-   Top cladding 44 height=208 nm.-   Layers 42,43, and 44 width=40 μm.-   All ridges height=100 nm.-   All ridges width=6 μm.

The used materials (silica for the ridges and for the bottom and topcladding, silica nitride for the core) are in relation with thetransmitted wavelengths. In particular, the multi/demultiplexer deviceof FIG. 3 is able to separate four channel at therespective wavelengthsof 810, 830, 850, 870 nm. At different wavelengths it is possible to useother materials, for example gallium arsenide, indium phosphatealuminum.

Each filter is centered on one of said transmission wavelengths. Inparticular, the first filter 452 transmits the signal at 810 nm andreflects the other wavelengths. The second filter 462 transmits thesignal at 830 nm and reflects the other wavelengths. The third filter472 transmits the signal at 850 nm and reflects the other wavelengths.The fourth filter 482 transmit the signal at 870 nm and reflects theother wavelengths.

The simulation regards the second filter 462 of the device. The otherfilters may be designed with a substantially similar method. In FIG. 8 aschematic view of the filter is illustrated.

The filter comprises ten defects DF1–DF10 and thirty-six cuts. Inparticular, the filter is symmetric with respect to a vertical axes Y′(dimension of D1 equal to dimension of D10, dimension of D2 equal todimension or D9, etc . . . ). The structure comprises in series a firsttransmissive couple (first defect DF1), two reflective couples, a secondtransmissive couple (second defect DF2), three reflective couples, athird transmissive couple (third defect DF3), three reflective couples,a fourth transmissive couple (fourth defect DF4), three reflectivecouples, a fifth transmissive couple (fifth defect DF5), threereflective couples, a sixth transmissive couple (sixth defect DF6),three reflective couples, a seventh transmissive couple (seventh defectDF7), three reflective couples, a eighth transmissive couple (eighthdefect DF8), three reflective couples, a ninth transmissive couple(ninth defect DF9), two reflective couples, a tenth transmissive couple(tenth defect DF10).

The dimension of the cuts and the defects are the following.

Width of cuts d_(cuts)=102 nm.

Width of blocks between cuts other than defects D1, . . . D10″ d=204 nm.

Width of defects D1 and D10 is d₁=1725 nm.

Width of defects D2 and D9 is d₂=2000 nm.

Width of defects D3 and D8 is d₃=3103 nm.

Width of defects D4, D5, D6 and D7 is d₄=3379 nm.

The spectral response of said filter is shown in FIG. 9. Said graph isthe spectral response for an incident optical beam directed for exampleperpendicular to the plane of the cuts. The graph of FIG. 9, is a goodpractical approximation of the spectral response of an angled filter,like the filters of FIG. 3 in which said incident beam has the samedirection of the ridge. In such case, the ridges of the waveguides arenot perpendicular to the plane of the cuts, but the ridge are directedin a way to guide the reflected beam in the next waveguide.

The central wavelength of the pass band PB′ is substantially around 830nm; this means that a channel at a such wavelength is transmitted bysaid filter.

The multi/demultiplexer of the present invention is a bi-directionaldevice. The signals at different wavelengths may travel along thewaveguides in both directions. Moreover, the filters are bi-directionalfilters. In an optical local area network bi-directional signals mayadvantageously be provided. For example, telephone and/or Internetdigital or analog signals need bi-directional devices. In the example afour channel multi/demultiplexer device is shown. Equivalently, morechannels may be added. In particular a fifth channel, for example amonitor signal, in a opposite direction may be used. In this case afurther waveguide (ridge) and a further filter on the substrate areprovided.

The device of present invention is realized on a semiconductorsubstrate. Advantageously, on the same substrate it is possible torealize electronics circuits. The substrate may comprise advantageouslyboth optical devices and electronic devices; thus, the present inventionprovides a compact device and with reduced dimensions.

The present invention may provide many configurations of the opticalmulti/demultiplexer device. In FIG. 10, for example, an alternativescheme of the multiplexer device 4′ is shown. In the example said deviceis able to separate a CDWM signal having four channels respectively at810 nm, 830 nm, 850 nm, and 870 nm. In particular, the device comprisesa first waveguide 45′ a second waveguide 46′ a third waveguide 47′ and afourth waveguide 48′. On said first waveguide a first filter 451′ and asecond filter 452′ are formed. On said second waveguide 46′ a thirdfilter 461′ is formed. At the end of first waveguide a CWDM signal isinput. The first filter 451′ transmits the channels at 850 nm and 870nm, and reflects the two remaining channels into the second waveguide46′. The second filter 452′ transmits the channel at 850 nm to an outputand reflects the channel at 870 nm to said fourth waveguide 48′ and to afurther output. The third filter transmits the 830 nm channel to afurther output and reflects the 810 nm channel to said third waveguide47′ and to a further output. In this embodiment the first filter is ableto transmit a band which comprises two channels (from 840 to 880 forexample). By means of the present invention it is possible to providefilters with an extremely large band; said large band may comprise morethan one channel.

1. Optical wavelength selective filter realized on an optical waveguide,said filter being able to transmit a first predetermined band ofwavelengths (PB′) and to reflect a second predetermined band (SB′) andcomprising a plurality of transmissive couples providing said firstpredetermined band and a plurality of reflective couples providing saidsecond predetermined band disposed in series to each other, eachtransmissive couple and each reflective couple comprising a first zonewith a high refractive index and a second zone with a low refractiveindex adjacent to each other in a way to form a structure with high gapmodulated refractive index, the structure comprising a firstsubstructure and a second substructure in the direction of propagationof the light, wherein the first substructure comprises at least a firstand a second transmissive couple in the direction of propagation of thelight, the width of the zone with high refractive index of the secondtransmissive couple being higher than the width of the zone with highrefractive index of the first transmissive couple.
 2. Optical wavelengthselective filter as claimed in claim 1, in which said zones with a lowrefractive index are realized by means of a plurality of transverse cutsprovided on said waveguides and said zones with a high refractive indexare blocks formed between two consecutive cuts.
 3. Optical wavelengthselective filter as claimed in claim 2, wherein each couple comprisesone cut and one block.
 4. Optical wavelength selective filter as claimedin claim 1, wherein between a first transmissive couple and a secondtransmissive couple at least a reflective couple is disposed.
 5. Opticalwavelength selective filter as claimed in claim 1, wherein the secondsubstructure comprises at least two transmissive couples having thezones with high refractive index decreasing in width in the direction ofpropagation of the light.
 6. Optical wavelength selective filter asclaimed in claim 1, wherein in the first sub structures the zones withhigh refractive index of the plurality of transmissive couples increaseor are equal in width in the direction of propagation of the light. 7.Optical wavelength selective filter as claimed in claim 5, wherein inthe second substructure: the zones with high refractive index of theplurality of transmissive couples decrease or are equal in width in thedirection of propagation of the light.
 8. Optical wavelength selectivefilter as claimed in claim 4, wherein in said first substructureconsecutive reflective couples between a transmissive couple and anotherincrease are equal in number in the direction of propagation of thelight.
 9. Optical wavelength selective filter as claimed in claim 4,wherein in the second substructure consecutive reflective couplesbetween a transmissive couple and another decrease are equal in numberin the direction of the propagation of the light.
 10. Optical wavelengthselective filter as claimed in claim 1 wherein said reflective coupleshave the same width.
 11. Optical wavelength selective filter as claimedin claim 1, wherein the structure has a first and a second half, thesecond half having a same configuration about the number of transmissivecouples and reflective couples, and about their disposition in sequence.12. Optical wavelength selective filter as claimed in claim 11, whereinsaid second half of the structure is symmetric with respect to saidfirst half.
 13. Optical wavelength selective filter as claimed in claim1, wherein in said reflective couples the sum of the width of said zonewith low refractive index and the width of said zone with highrefractive index is substantially an even multiple of a quarter of thecentral wavelength of said second predetermined band (SB′).
 14. Opticalwavelength selective filter as claimed in claim 1, wherein in saidtransmissive couples the sum of the width of said zone with lowrefractive index and the width of said zone with high refractive indexis substantially an odd multiple of a quarter of the central wavelengthof said first predetermined band (PB′).
 15. Optical multi/demultiplexerdevice, comprising: a substrate; a plurality of waveguides on saidsubstrate; and at least one wavelength selective filter according to anyof claims 1 to 14, realized on one of said plurality of waveguides. 16.Optical multi/demultiplexer device as claimed in claim 15, wherein, atleast one of said plurality of waveguides comprises a ridge.
 17. Opticalmulti/demultiplexer device as claimed in claim 15, wherein saidsubstrate is a semiconductor substrate.
 18. Method for realizing anoptical wavelength selective filter on a substrate comprising thefollowing steps: providing a waveguide in a optical transmissivematerial on said substrate; forming in said waveguide a plurality oftransmissive couples providing a first predetermined transmissionwavelength band and a plurality of reflective couples providing a secondpredetermined reflection wavelength band disposed in series to eachother, each transmissive couple and each reflective couple beingprovided with a first zone with a high refractive index and a secondzone with a low refractive index adjacent to each other in a way to forma structure with modulated refractive index; and characterized in thatit further comprises the step of providing the structure with at least afirst and a second transmissive couple in the direction of propagationof the light in which the width of the zone with high refractive indexof the second transmissive couple is higher than the width of the zonewith high refractive index of the first transmissive couple.
 19. Opticalwavelength selective filter realized on an optical waveguide, saidfilter being able to transmit a first predetermined band of wavelengths(PB′) and to reflect a second predetermined band (SB′) and comprising aplurality of transmissive couples providing said first predeterminedband and a plurality of reflective couples providing said secondpredetermined band disposed in series to each other, each transmissivecouple and each reflective couple comprising a first zone with a highrefractive index and a second zone with a low refractive index adjacentto each other in a way to form a structure with modulated refractiveindex, the structure comprising a first substructure and a secondsubstructure in the direction of propagation of the light, wherein thefirst substructure comprises at least a first and a second transmissivecouple in the direction of propagation of the light, the width of thezone with high refractive index of the second transmissive couple beinghigher than the width of the zone with high refractive index of thefirst transmissive couple.